Displays comprising a plurality of light-emitting elements, and in particular solid-state organic light-emitting diode (OLED) image display devices, are of great interest as flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the light-emitting elements change with time or use, as the organic materials in the device age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials may age at different rates, causing differential color aging and a display whose white point varies as the display is used. If some light-emitting elements in the display are used more than other, spatially differentiated aging may result, causing portions of the display to be dimmer than other portions when driven with a similar signal. In particular, this may occur when the screen displays a single graphic element in one location for a long period time. Such graphic elements can include stripes or rectangles with background information, for example such as news headlines and sports scores, network logos, and the like. Differences in signal format are also problematic.
Computer monitors typically employ screen savers that are automatically displayed when no user interaction has been detected for a predetermined period of time. The screen savers may either blank the screen or employ a variable image signal to prevent excessive aging, in particular localized aging. However, for entertainment applications, user interaction may be infrequent and localized aging can become a problem for displays that are susceptible to this problem.
Television broadcasts may have a variety of signal variations, even when a static image is conveyed. For example, transmissions are subject to a variety of noise factors that can slightly change the signal. Any digitization of the analog signal may result in slight variations that result from these variations, as well as inherent noise in the digitization process. Moreover, a live broadcast of a static scene may have slight variations in camera location that will result in similar variability. Hence, two frames that are ostensibly identical, when processed within a consumer's television receiver, will have minor differences and a comparative method for detecting static images that relies on an identical match may fail inappropriately. In other cases, a scene may be largely static but have one small area that varies significantly. For example, a web page may have completely static content except for a clock or continuously updated text in one small area. Such a scene may also be problematic with respect to avoiding burn-in in a display. In yet another case, a single frame interruption of a static scene broadcast (for example with an intermittent electromagnetic interference in the broadcast system) may be incorrectly interpreted as a cessation of a static image broadcast. Likewise, horizontal or vertical sync variability may produce a similar, deleterious effect. Digital signals may have other problems with signal corruption, for example blocking errors or decompression faults, or a broadcast signal may be interrupted.
The general problem of regional brightness differences due to icon burn-in of specific areas due to video content has been addressed in the prior art, for example by U.S. Pat. No. 6,856,328 B2 entitled, “System and method of displaying images” Logos may be present in images transmitted by television stations. These logos are often present in the corners of an image for a long time. They do not move and may comprise saturated colors. This results in burn-in effects in emissive displays because the logos provide the same display load at the same location for a relatively long period of time. The burn-in effect can be prevented by detecting the logos in the corners of the image and reducing their intensity to the average display load. Alternatively, US20050246657 A1 entitled “Video display arrangement including image processing circuitry for protecting display and method of protecting a video display” describes a video display arrangement that includes a display and a receiver. The receiver includes video imaging processing circuitry, the circuitry including a video formatter adapted to transmit formatted active image signals, a display buffer having a video display memory for temporarily storing the active image signals and transferring the active image signals to a display, a comparator for comparing one or more blocks of the video display memory for changing content over time and sending a static content warning signal to the video formatter when content in the one or more blocks remains static beyond a predetermined static content period, and program storage including one or more programs adapted to cause the video formatter to transfer a changing content image to the display buffer after receiving the static content warning signal. However, this design requires the use of a frame-store to store images for comparison. Such frame stores, particularly for analog signals, are expensive. Moreover, the comparator is not robust in the face of minor and inconsequential signal changes and the use of content-changing signals to individual blocks may not be acceptable to users.
U.S. Pat. No. 6,313,878 B1 entitled “Method and structure for providing an automatic hardware-implemented screen-saver function to a display product” describes a hardware-implemented screen-saver that prevents burn-in of an image displayed on a screen of a display product by automatically reducing the video gain, and therefore the contrast, of the image when the portion of the image within a two-dimensional detection window has changed by less than a predetermined amount for a predetermined period of time. A lack of change of the incoming video signal of the image is detected and used to invoke a reduction in contrast of the image displayed on the display product. This allows the image to remain visible, yet reduces the possibility of burn-in of the image in the screen of the display product. This disclosure describes the use of voltage averaging circuits and checksums, thereby mitigating the need for a frame-store. It also describes the use of user-defined windows for selecting a portion of an image signal. However, this embodiment may require user interaction, be limited to a single sampling window, and fail to be robust in the presence of minor signal variations.
There is a need, therefore, for an improved method of detecting image signals that can cause localized burn-in for a display having a plurality of light-emitting elements having outputs that change with time or use.